Improved coverage in wireless communication systems such as Third Generation Partnership Project (3GPP™) LTE is sought after for various reasons. Coverage improvements may call for an increase in practical gain of various physical channels such as the Physical Downlink Control Channel (PDCCH). The LTE protocol currently being developed and implemented provides a high-speed wireless communication network for use by devices such as mobile phones and data terminals. The LTE protocol itself is detailed in various documents, for example as published by the 3rd Generation Partnership Project (3GPP).
Hybrid Automatic Repeat Request (HARQ) processes proposed for LTE transmit blocks of data along with error detection and forward error correction (FEC) bits. In incremental redundancy HARQ, if the errors in a received data block cannot be corrected, the transmitter is informed via Negative Acknowledgement (NACK) and the data block is retransmitted. The retransmission is coded differently from the previous transmissions and retransmissions of the same data block; that is in accordance with a different redundancy version (RV). The receiver may then combine different redundancy versions to improve the probability of successfully decoding the data block. This type of HARQ effectively lowers the coding rate with each retransmission. HARQ effectively improves coverage, however due to signalling and processing delays, only one redundancy version can be transmitted every 8 ms.
TTI bundling has been proposed as a modification to the HARQ mechanism, in order to provide improved coverage without dramatically increasing latency and signalling overhead. A detailed overview of TTI bundling can be found in “HARQ Operation in case of UL Power Limitation,” Ericsson, June 2007, 3GPP TDoc R2-072630. Essentially, rather than waiting for a NACK before transmitting the next redundancy version of a data block, several redundancy versions are transmitted in consecutive transmit time intervals (TTIs). The HARQ feedback is sent after the last redundancy version of the data block is received. Current implementations specify transmission of four redundancy versions at a time. To preserve backward compatibility, further retransmission (if triggered due to a NACK) is delayed until the 16th TTI after the first redundancy version was sent.
Increasing the TTI bundle size beyond 4 RVs is a possible way to improve coverage. For example, a 3rd Generation Partnership Project (3GPP) work item entitled “Updated SID on: Provision of low-cost MTC UEs based on LTE” (TSG RAN meeting #57, Chicago, USA, September, 2012, RP-121441), relates to a new study requiring a +20 dB improvement in coverage for LTE systems. To obtain such a +20 dB improvement through TTI bundling would require a TTI bundle size of 400 RVs.
However, it has been recognized by the inventors that the current TTI bundling methods potentially result in inefficiencies if certain messages are lost or corrupted. In particular, if the User Equipment (UE) fails to decode a downlink grant message which notifies the UE that that a large TTI bundle is forthcoming for it, then that UE will not attempt to decode the TTI bundle. However, currently there is no mechanism to notify the eNB that the UE is not receiving the TTI bundle and hence the Evolved Node B (eNB) will transmit the TTI bundle in its entirety, thereby unnecessarily consuming spectral resources.
Therefore there is a need for a method and system for implementing TTI bundling that is not subject to one or more limitations of the prior art.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present technology. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present technology.